Digital Subscriber Line (DSL) is a data transmission technology using telephone twisted pairs as the transmission medium. xDSL is a combination of DSL technologies including High-speed Digital Subscriber Line (HDSL), Single-pair High-Speed Digital Subscriber Line (SHDSL), and Asymmetrical Digital Subscriber Line (ADSL). SHDSL is based on baseband transmission. Other xDSL technologies, which are based on passband transmission and use the Frequency-Division Multiplexing (FDM) technology, may coexist with Plain Old Telephone Service (POTS) in the same twisted pairs.
As higher and higher bands are used by xDSL based on passband transmission, the highband crosstalk has become a severe problem. FIG. 1 shows a method for solving the crosstalk between xDSL lines by using a Vectored Digital Subscriber Line (Vectored-DSL) technology in the prior art. In the downlink direction, x indicates signal vectors sent by Nx1 coordinated transceiver devices such as a Digital Subscriber Line Access Multiplexer (DSLAM); y indicates signal vectors received by Nx1 opposite devices such as a subscriber-side device; and n indicates Nx1 noise vectors. The following channel transmission matrix indicates a shared channel:
  H  =      [                                        h            11                                                h            12                                    ⋯                                      h                          1              ⁢              M                                                                        h            21                                                h            22                                    ⋯                                      h                          2              ⁢              M                                                            ⋮                          ⋮                          ⋱                          ⋮                                                  h                          N              ⁢                                                          ⁢              1                                                            h                          N              ⁢                                                          ⁢              2                                                ⋯                                      h            NN                                ]  
hij(1≦i≦N,1≦j≦N) indicates a crosstalk transfer function of pair j to pair i; hii(1≦i≦N) indicates the channel transfer function of pair i; and N indicates the number of pairs, that is, the number of subscribers. If a vector pre-encoder (represented by W) is used in a coordinated transceiver device, the signal vectors that an opposite device receives are calculated according to the following formula:{tilde over (y)}=HWx+n 
If the vector pre-encoder can make HW a diagonal matrix, for example, diag(H), the crosstalk may be cancelled. To cancel the crosstalk, channel estimation needs to be performed to obtain a channel transmission matrix.
While developing the present invention, the inventor found that signal errors are used to estimate channels in the prior art and devices are required to provide signal errors. Many devices running on a network, however, do not support this function. As a result, signal errors are not available for channel estimation and thus the crosstalk cannot be cancelled.